We are investigating the genetic control of DNA replication and DNA repair in Escherichia coli infected with bacteriophage T4. Three types of approaches are being used in these investigations: Studies on the function of the T4 phage rII gene are directed towards the identification of the different metabolic processes which are affected by the expression of this gene at different times after viral infection. Phage genes which are known to be involved in the control of viral DNA synthesis are being investigated for effects on the fidelity of base selection in DNA synthesis. Alterations in this fidelity result in alteration of mutation rates of viral genes. It is likely that DNA repair and replication are controlled by multienzyme complexes and that the proper formation of these complexes is dependent on the proper regulation of the synthesis of their components. Mutations are being isolated which result in abnormal regulation of enzyme synthesis and their effects on the control of in vivo DNA synthesis are being studied.